Electric motors are used in a variety of electrical equipment. In particular, they are used in various manufacturing equipment. For example, wafer stages positioning silicon wafers during photolithography and other semiconductor processing equipment utilize linear electric motors to position the wafer or other substrates.
A typical one-dimensional linear electric motor has a magnet track with pairs of opposing magnets facing each other. Within spaces between the pairs of opposing magnets, an armature moves. The armature has windings of a conductor which are supplied with an electrical current. When the electrical current is turned on, forces proportional to the product of the current and a local magnetic field are generated to cause the armature to move. When the armature is attached to a wafer stage, the wafer stage moves in concert with the armature.
In a multiphase motor, the armature has various windings grouped into phases. The phase groups are selectively pulsed with electric current to create a more efficient motor. As the armature moves within the magnet track as a first group of coils is pulsed, the first group moves out of its optimal position between the pairs of magnets. Then, it becomes more efficient to pulse a second group of windings. More phase groups are theoretically more efficient since a more even application of force and utilization of power input is maintained. However, each additional phase group complicates a timing of the pulses to the various phase groups. Presently, three-phase motors and armatures have gained favor in balancing these considerations.
There are several types of two-dimensional linear motors. U.S. Pat. No. 4,535,278, Two Dimensional Precise Positioning Device for use in a Semiconductor Manufacturing Apparatus, issued to Teruo Asakawa on Aug. 13, 1985 and incorporated by reference herein in its entirety, describes a two-dimensional motor. This motor has four identical coils placed in a plane near and parallel to an array of permanent magnets, which are arranged in a spaced apart rectangular pattern, with magnets of like polarity disposed along diagonals. On the side of the coils opposite this array is disposed either a second identical array, or a plate of ferromagnetic material which serves as a magnetic flux return path, to complete a magnetic circuit. The coils' dimensions are approximately equal to the pitch of the magnet array, and they are square in shape. Assuming the magnet arrays to lie in the horizontal plane, the magnetic fields are primarily vertical, as are the axes of the coils. Electric currents produced in the coils generate driving forces in the horizontal plane. These forces are primarily along the axes of the magnet array.
Another version of the motor is described in U.S. Pat. No. 4,555,650, Two-dimensional Driving Device for use in a Positioning Device in a Semiconductor Manufacturing Apparatus, issued to Teruo Asakawa on Nov. 26, 1985 and incorporated by reference herein in its entirety. This second version utilizes pairs of coils which are oriented vertically and may be placed between two horizontal arrays of permanent magnets. The magnet arrays are identical to those described above, except that corresponding magnets in the two arrays are arranged in opposition; i.e., the north poles in the top array are directly above the north poles in the bottom array, and similarly for the south poles. The coil pairs have their planes parallel, and a magnetically permeable member extends between them, linking their cores. This member provides a flux return path, so that the flux from one permanent magnet approximately beneath one of the coils can link both coils of the pair before entering the adjacent permanent magnet of opposite polarity approximately beneath the other coil. If currents of appropriate polarity are passed through the coils a horizontal force along the line between the coils is produced. Similar coil pairs oriented orthogonally can produce forces in the orthogonal direction, allowing for two-dimensional motion.
U.S. Pat. No. 4,654,571, Single Plane Orthogonally Movable Drive System, issued to Walter E. Hinds (Hinds) and incorporated by reference herein in its entirety, describes another two-dimensional motor. The Hinds motor has a horizontal planar array of permanent magnets with like magnetization of magnets along rows and columns and a set of four coils attached to a platform supported above an array by air bearings. The coils are arranged in quadrants with their active turns aligned parallel to the plane of the magnet array. Coils diagonally opposite one another are aligned in parallel. Sending an electrical current through such coil pairs generates a force along one of the array axes if the coils are placed where the magnetic fields from the magnets are essentially vertical. The plane of each coil is vertical, and the length of the active turn part of the coil is a multiple of the magnet array pitch.
A relatively new application of two-dimensional linear motors is precision stage control for electron beam machines which are used for lithography, metrology, and inspection. For these applications, the stray magnetic fields from both the permanent magnets and the coils can seriously perturb the electron beam. For this reason a configuration which minimizes the stray fields is desirable.
What is needed is a linear motor providing more stable horizontal forces with higher electrical efficiency and requiring a simpler control system. Moreover it should have lower stray magnetic fields making it more suitable for electron beam applications.